Likelihood of occurrence

Criticality Mnalysis. The criticaUty assessment provides a figure-of-merit for each failure mode. This figure of merit is based on the likelihood of occurrence of the failure mode (Occ), the criticaUty (severity) of the failure mode on system performance (Sev), and the detectabiUty of the failure mode by the user prior to occurrence (Det). 

Considerable effort has been directed to determining the causes of connection failutes and to learning how to minimize the likelihood of occurrence. Acceptable failute rates range from <1 in 10 operating hours for contacts in air-frame (31) electrical systems and in some telecommunications equipment, to 100—1000 in 10 operating hours in instmments, to even larger rates for contacts in many consumer products. A failute is defined as exceedance of contact resistance, which can be as Httle as twice the initial contact resistance, that causes circuit malfunction. The required lifetimes of connectors may be >20 yr, although most required appHcation times ate shorter (see Materials reliability). 

Mitigation Reducing the risk of an accident event sequence by taking protective measures to reduce the likelihood of occurrence of the event, and/or reduce the magnitude of the event and/or minimize the exposure of people or property to the event. 

The frequency analysis step involves estimating the likelihood of occurrence of each of the undesired situations defined in the hazard identification step. Sometimes you can do this through direct comparison with experience or extrapolation from historical accident data. While this method may be of great assistance in determining accident frequencies, most accidents analyzed by QRA are so rare that the frequencies must be synthesized using frequency estimation methods and models. 

The expression for the likelihood of occurrence of an event or an event sequence during an interval of time or the likelihood of the success or failure of an event on test or on demand. By definition, probability must be expressed as a number ranging from 0 to 1... 

Mitigation The lessening of die risk of an accidental event. A sequence of action on the source in a preventive manner by reducing the likelihood of occurrence of the event, or in a protective manner by reducing the magnitude of the event and for the exposure of local persons or property. 

The paired comparisons method (NUREG/CR-3688) is a structured expert judgment method in which human errors are compared in pairs. By combining the judgments of the group of experts, the errors arc arranged in order of likelihood of occurrence, of the human errors considered, they can be used as "anchor points list. Documentation requirements are given in Table 4.5-8. 

When performing human reliability assessment in CPQRA, a qualitative analysis to specify the various ways in which human error can occur in the situation of interest is necessary as the first stage of the procedure. A comprehensive and systematic method is essential for this. If, for example, an error with critical consequences for the system is not identified, then the analysis may produce a spurious impression that the level of risk is acceptably low. Errors with less serious consequences, but with greater likelihood of occurrence, may also not be considered if the modeling approach is inadequate. In the usual approach to human reliability assessment, there is little assistance for the analyst with regard to searching for potential errors. Often, only omissions of actions in proceduralized task steps are considered. 

The SRK model can also be used as part of a approach for the elimination of errors that have serious consequences proactive for the plant. Once specific errors have been identified, based on the SRK model, interventions such as improved procedures, training or equipment design can be implemented to reduce their likelihood of occurrence to acceptable levels. This strategy will be discussed in more detail in Chapter 4. 

For errors with serious consequences and/or high likelihood of occurrence, develop appropriate error reduction strategies. 

The Chemical Process Industry (CPI) uses various quantitative and qualitative techniques to assess the reliability and risk of process equipment, process systems, and chemical manufacturing operations. These techniques identify the interactions of equipment, systems, and persons that have potentially undesirable consequences. In the case of reliability analyses, the undesirable consequences (e.g., plant shutdown, excessive downtime, or production of off-specification product) are those incidents which reduce system profitability through loss of production and increased maintenance costs. In the case of risk analyses, the primary concerns are human injuries, environmental impacts, and system damage caused by occurrence of fires, explosions, toxic material releases, and related hazards. Quantification of risk in terms of the severity of the consequences and the likelihood of occurrence provides the manager of the system with an important decisionmaking tool. By using the results of a quantitative risk analysis, we are better able to answer such questions as, Which of several candidate systems poses the least risk Are risk reduction modifications necessary and What modifications would be most effective in reducing risk ... 

A risk estimate indicates Uie likelihood of occurrence of the different types of health or enviroinnental effects in exposed populations. Risk assessment should include both liuimn health and environmental evaluations (i.c., impacts on ecosystems). Ecological impacts include actual or potential effects on plants and animals (other than domesticated species). The number produced from the risk characleriznlion, representing the probability of adi crse... 

Individual Risk - This provides a measure of tlie risk to a person in tlie vicinity of a liazard/accideiit, including tlie nature of the injury or otlier undesired outcomes, and tlie likelihood of occurrence. Individual risk is generally expressed in terms of a likelihood or probability of a specified undesired outcome per unit of time. For example (as indicated above), tlie... 

Each cell in tlie matrix (Table 18.4.2) is assigned a risk ranking as indicated by the letters. In this approach, an A level risk corresponds to a very severe consequence with a high likelihood of occurrence. Action must be taken, and it must be taken promptly. At tlie other end of the scale, a E level risk is of little or no consequence witli a low likelihood of occurrence, and no action is needed or justified. For example, a level C risk might warrant mitigation witli engineering and/or administrative controls or may represent risks tliat are acceptable with controls and procedures. 

Risk assessment, an obvious precursor to risk management, first identifies a hazard and then qtrantifies the likelihood of occurrence (hazard assessment) and the impact (expostrre assessment) associated with each hazard event. 

It is also apparent that the sequence of monomer units in an ideal copolymer must necessarily be random. That is to say, the likelihood of occurrence of an Mi unit immediately following an M2 unit is the same as for an Mi to follow an Mi unit. The probability of either unit at any place in the chain is always equal to its mole fraction in an ideal copolymer. ... 

As QRA involves an explicit consideration of both the frequency and consequences of accidental events, it promotes a balanced perspective in the assessment and control of major hazards. QRA is valuable in reconciling divergent opinions that may arise between those who, irrespective of consequences, tend to dismiss the risk on the grounds of its remoteness and those who tend to concentrate on the possible consequences irrespective of their likelihood of occurrence. 

Qualitative frequency evaluation involves defining broad categories of event frequency, which can be used to assess the likelihood of occurrence of a specific incident outcome (consequence). These categories cover a full spectmm of frequencies, from those representing events that are likely to those that are highly unlikely. Definitions of likelihood categories vary, but Table 5.4 presents a typical list and definitions. 

Top event the unwanted event or incident at the "top" of a fault tree that is traced downward to more basic failures using logic gates to determine its causes and likelihood of occurrence. 

The essence of the ASTM E 2012 approach is to determine incompatibility scenarios that could foreseeably occur by examining all possible binary combinations. It may be necessary to review a process by using a systematic method such as a process hazard analysis (PHA) to identify all incompatibility scenarios that have a significant likelihood of occurrence and severity of consequences. The same review can then be used to evaluate whether adequate safeguards exist or whether further risk reduction is warranted. 

Risk assessment starts with risk identification, a systematic use of available information to identify hazards (i.e., events or other conditions that have the potential to cause harm). Information can be from a variety of sources including stakeholders, historical data, information from the literature, and mathematical or scientific analyses. Risk analysis is then conducted to estimate the degree of risk associated with the identified hazards. This is estimated based on the likelihood of occurrence and resultant severity of harm. In some risk management tools, the ability to detect the hazard may also be considered. If the hazard is readily detectable, this may be considered a factor in the overall risk assessment. Risk evaluation determines if the risk is acceptable based on specified criteria. In a quality system environment, criteria would include impact on the overall performance of the quality system and the quality attributes of the finished product. The value of the risk assessment depends on how robust the data used in the assessment process is judged to be. The risk assessment process should take into account assumptions and reasonable sources of uncertainty. Risk assessment activities should be documented. 

Principle 1 List potential hazards, assess the likelihood of occurrence, and identify the preventive measures for their control... 

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